In recent years, with advances in data compression technologies, propagation of high-speed networks, and the like, high-value content with high quality (for example, content with a 4 k resolution) has been distributed as digital data through various media. Further, for example, information devices such as personal computers have high performance, and thus it is easy for illegal devices to use content illegally. For this reason, there is more of a demand than ever to reinforce protection of content which is digital data. High-value content makers and suppliers are assumed not to permit digital transmission when security is not reinforced.
In order to reinforce security of content protection systems, changing encryption schemes to be used is considered.
For example, key generation devices that vary the lengths of keys to be used in accordance with requested service security levels by generating a plurality of public keys corresponding to a plurality of secret keys from single secret keys of users and storing the public keys in certificates (for example, see Patent Literature 1).